Extruding a coating of a polyolefin or blends of polyolefins onto a substrate, such as paper or aluminum foil, to form an extrusion coated substrate, is well known in the art. Extruding multiple layers of polymers including polyolefins as well as other materials in a process known as co-extrusion is also well known. Various polyethylenes and blends of polyethylenes have been used widely as extrusion coating compositions. Such materials have also been used in coextrusion processes as the layer against the substrate so as to adhere the coating to the substrate. Unfortunately, the use of polyethylene-based coatings has several drawbacks. For example, such materials frequently lack the desired level of adhesion to typical extrusion coated substrates, especially when extrusion coating is carried out at low temperatures and/or at extremely high coating speeds. In addition, polyethylene-based coatings typically require extrusion coating temperatures that generate excessive odor and are not compatible, in co-extrusions, with some heat sensitive polymers.
Low temperature extrusion coating processes are useful when applying coextrusions with other polymers which are temperature sensitive. Low temperature extrusion coating processes are also useful when employing substrates which are temperature sensitive. Low temperature extrusion coating processes are also useful when applying polymeric coatings which contain additives which are temperature sensitive. Standard extrusion coating materials, particularly, polyethylene employed at low extrusion temperatures, are not capable of adhering to the substrate with the desired degree of adhesion.
High extrusion coating speeds are desirable so as to enable economically attractive operation of extrusion coating equipment. High extrusion coating speeds are also desirable so as to permit the preparation of very thin extrusion coatings.
Attempts to improve the adhesion to the substrate and coating speeds for extrusion and coextrusion are disclosed in U.S. Pat. No. 5,112,424, copending application Ser. No. 578,036 filed Sept. 5, 1990, and copending application Ser. No. 681,801 filed Apr. 5, 1991. These compositions and processes improve the adhesion of particular polyethylene compositions to the substrate by the addition of particular tackifying resins. To accomplish this good adhesion to the paper substrates, a high loading of up to 15 weight percent of tackifier is sometimes required. The tackifier, while promoting adhesion to the substrate, also increases the adhesion to the chill roll surface in extrusion coating operations, particularly when using a high gloss chill roll surface. The adhesion to the chill roll provides poor chill roll release. Sometimes the adhesion forces between the tackified composition and the chill roll exceed the adhesion forces between the tackified composition and substrate, and the polyethylene is pulled away from the substrate as the coated structure is pulled from the chill roll surface. Sometimes the adhesion force between the tackified polyethylene composition and substrate are so high that a destructive bond at the interface is formed while adhesion forces between the tackified polyethylene composition and chill roll are also high resulting in the substrate, when made of paper, being ruptured internally as the coated structure is pulled from the chill roll. In either case, poor bond between the coating and substrate or substrate rupture is undesirable.
U.S. Pat. No. 4,526,919 discloses the use of fatty acid amides containing 16 to 40 carbon atoms as chill roll release agents in polypropylene extrusion coating.
It would be very desirable to be able to reduce the adhesion between the tackified polyethylene composition and chill roll without detracting from the adhesion forces required to bond the polyethylene to the substrate at low temperature extrusion coating. It would also be very desirable to be able to reduce this adhesion to the chill roll while maintaining the polyethylene's good surface qualities such as flame acceptance, corona priming, and heat seal ability.